Solar bubble pumps have been described in U.S. Pat. Nos. 4,552,208 issued Nov. 12, 1985 and 4,607,688 issued Aug. 26, 1986 to Sorensen. Using the bubble pump principle hot liquid can be transferred from one elevation to a lower elevation without the need for an externally powered pump. Another design is described in Haines et al U.S. Pat. No. 4,478,211 which shows a system in which hot liquid is tranferred from a higher level to a lower level and in which the vapor from the bubble is injected into the gas space above the liquid in the riser tubes. Heretofore, however, bubble pumps have not been used in series to amplify the pressure gradient therebetween in such a way that useful kinetic energy can be derived therefrom. It is believed that heretofore it has not been possible to control the boiling temperature in the bubble pump so that balance between a series of bubble pumps could be achieved. Unless the pumps are balanced, some will become flooded and some will become underfilled resulting in a decreased pressure differential across the system.